feat: Add VideoSlice node with lazy operations on VideoInput

- Add VideoOp base class and SliceOp in _input/video_types.py
- Add sliced() method to VideoInput that returns a copy with operation appended
- Each subclass applies operations in get_components() and get_frame_count()
- After materialization, VideoFromFile delegates to internal VideoFromComponents
- Add VideoSlice node that uses video.sliced(start_frame, frame_count)
- Add tests for SliceOp, sliced() behavior, and materialization

comfy_api/latest/_input_impl/__init__.py

@@ -1,7 +1,8 @@
 from .video_types import VideoFromFile, VideoFromComponents
+from .._input import SliceOp
 
 __all__ = [
-    # Implementations
     "VideoFromFile",
     "VideoFromComponents",
+    "SliceOp",
 ]